This top-level builder script will build as many projects as are
possible to build on the current platform. For example, iOS and macOS
projects can only be built on macOS; Windows and parts of Developer can
only be built on Windows.
Relates-to: #6268
Add `builder_describe_platform` function to builder.inc.sh, associated
documentation and unit test, and use it in Developer and Common build
scripts.
Relates-to: #11755
Note: there is a bit of potential confusion about the difference between
/resources/builder.inc.sh (the full implementation for builder scripts),
and /resources/build/builder.inc.sh (the source script that builder
scripts should always use).
This allows us to make assumptions that will always be true for builder
scripts that may not be true for other scripts, such as setting base
folder.
